Oxy-fuel combustion along with IGCC and a number of other technology options form a broad portfolio of innovative technology options commonly referred to as Low Emission Coal Technologies (LECTs). Approximately 70% of the future cuts in global greenhouse gas (GHG) emissions are estimated to be due to implementation of LECTs. Among these, oxy-fuel combustion is particularly attractive because of its inherent ability for in-situ separation of CO2. However, oxy-fuel combustion requires oxygen and, thereby, an air separation unit (ASU) to function effectively. Moreover, a number of major issues, chiefly among them the heat transfer limitations, ineffective reactor designs, gas cleaning, and the high energy demand of ASUs, need to be resolved before oxy-fuel technology can be deployed worldwide on a truly commercial basis.
While many of these issues can be effectively resolved given the current level of maturity in the field of combustion and process engineering, reducing the energy footprint and capital cost of ASUs is a more challenging problem requiring a radically new solution.
Oxygen is commonly produced at industrial scales by air separation using cryogenic distillation and adsorption based technologies. Advanced technologies such as membrane separation (e.g. ion-transport membrane, ITM) and in-situ air separation are also being developed for small-volume point-of-use oxygen generation. Generally, cryogenic systems are employed in large-scale production of high-purity oxygen while adsorption systems are employed at the lower end of the production scale and for lower oxygen purities. In cryogenic separation air is liquefied at very low temperatures and, hence, oxygen is selectively removed from the air by distillation. Cryogenic air separation involves a number of key steps, including: (i) air compression; (ii) air liquefaction, and (iii) distillation to separate oxygen from other gases. The process is very effective because it can be accurately controlled by adjusting the pressure and temperature. But cryogenic processes are generally expensive owing to the energy intensity of the air compression process. Considerable efforts have been made in recent years to improve the mechanical and thermodynamic efficiencies of compressors for air separation applications. However, even an ideal compressor with a 100% adiabatic efficiency still requires a significant amount of power to compress large volumes of air to sufficiently high pressures (≈36 bar).
Conventional adsorption methods (e.g. pressure swing adsorption, PSA) of producing oxygen rely on selective physical adsorption of O2 (or N2) on the internal pores of a high surface area adsorbent material. Both carbon and zeolite molecular sieves are commonly used in PSA and vacuum-PSA (VPSA) type air separation plants. Adsorption plants operate in a cyclic manner with the basic steps being adsorption (i.e. O2 or N2 removal from air) and regeneration (i.e. release of O2 or N2 form the saturated adsorbent material). Similar to the cryogenic methods, air compression is a key step in the adsorption based air separation methods and as such the specific power consumptions of PSA and VPSA plants are not much lower than their cryogenic counterparts.
Membranes rely on a barrier film to separate O2 from air. The film allows selective permeation of O2 and can be made from a host of different materials including polysulphone and acetate. More advanced membrane systems, such as ITMs, allow the rapid transfer of oxygen ions, achieving fluxes which are orders of magnitude higher than polymeric membranes. Perovskite membranes (e.g. La1-xAxCol1-yFeyO3-I) have been also employed in membrane reactors for in-situ oxygen generation. Oxygen in this process though fully reacts with a fuel leaving no excess oxygen for collection as a product. Membranes are generally modular and can be replicated to satisfy the throughput requirements. This however generates a degree of complexity in terms of system integration and installation. Membranes have been in commercial use for several decades but much of their past applications have been in liquid-liquid and liquid-solid separation. The use of membranes for large volumetric gas flow rates, such as those in air separation, has not been demonstrated yet. Membrane systems also suffer from high cost of manufacture.
Other methods for air separation (i.e. non-cryogenic, non-adsorption) have also been developed in the past. The earliest example is the thermal cycling of alkaline manganates for air separation which was demonstrated for a short period in 1866 as a commercial operation. Processes based on absorption/desorption of barium oxide have been also investigated by several researchers. The process was generally difficult to operate since desorption of oxygen had to be carried out under a strong vacuum. A more recent air separation method called “MOLTOX” was developed by Erickson in 1980s. The process was carried out by temperature swing absorption of oxygen from air using alkali metal nitrates and nitrites. The process did not lead to any commercial applications due to operational difficulties associated with handling molten salts.
Electrolysis and thermo-chemical cycles for water splitting have been also studied for hydrogen and oxygen production. Over 250 thermo-chemical cycles have been reported in the literature although only a few have proven to be economically feasible. This is not surprising given that the water splitting reaction is thermodynamically feasible at temperatures in excess of 1600° C., requiring a complex and expensive reactor system driven by solar energy. Electrolysis of water is energy intensive too.
Integrated SOFC-E systems (solid oxide fuel cell electrolyser) have been recently proposed to resolve this drawback. The throughput of such systems, though, is very low making them most suitable for small-scale on-site applications.
Given the above background, cryogenic air separation systems appear to be the only practical option for oxy-fuel applications. However, a cryogenic air separation unit with a typical specific power consumption of about 0.4 kWh/[m3 O2] may consume between 10% and 40% of the gross power output of the oxy-fuel plant. Cryogenic ASUs also typically constitute 40% of the total equipment cost or about 14% of the total plant cost.
Clearly there is therefore a need for a more simple and cost effective air separation technology with much smaller energy footprint and lower capital cost than conventional and emerging membrane and/or adsorption based air separation methods.
The present invention uses a chemical looping air separation process fully integrated with the processes of a large-scale oxy-fuel power generation plant to achieve this outcome.